Hollywood director Brett Ratner has produced a powerful new documentary that uncovers an unknown chapter in film — and world — history: Alfred Hitchcock directed a harrowing documentary about the Holocaust for the British military after World War II, but the project was suppressed by the British government.

After WWII ended in Europe, the British government trained a group of soldiers as cameramen to document the atrocities uncovered by ­Allied soldiers in Nazi concentration camps.

The graphic, firsthand footage was to be used as evidence against Germany, and Alfred Hitchcock was asked to direct the movie, called “German Concentration Camps Factual Survey.”

Brett Ratner attends the New York premiere of “Night Will Fall.”Getty Images

Hitchcock agreed and began in 1945. But just before his film was ultimately completed, the British government shut it down as Winston Churchill left office and the UK sought to ally with Germany against the Soviet Union.

Ratner’s doc “Night Will Fall,” about the lost film’s history — which features some of Hitchcock’s original footage as well as interviews with the men who shot it — screened at the Museum of Jewish Heritage on Thursday.

An emotional Ratner appeared near tears introducing the project. “The saddest thing is that the survivors [of the Holocaust] here today have to watch events 70 years later [like] the atrocities in France,” he said, referring to the Kosher-market attack there.

Directed by Andre Singer and narrated by Helena Bonham Carter, “Night Will Fall” describes how Allied forces trained soldiers to use the cameras, and how the results were hundreds of hours of firsthand footage of inconceivable suffering by Jews and other groups at Bergen-Belsen, Dachau, Auschwitz and more camps.

HBO will air the film on Jan. 26 and Jan. 27 to coincide with the 70th anniversary of Auschwitz’s liberation.